Magneto-optical disk memories are put to practical use as rewritable optical disk memories.
However, the magneto-optical disk memories suffer from a disadvantage in rewriting. Namely, it takes time to rewrite information. When writing information again in a portion containing information, there is a need to erase the previously recorded information before performing the rewriting.
In order to overcome such a disadvantage, overwriting by magnetic-field modulation is used as a method for overwriting without performing erasing before rewriting.
In the case of using this method, however, the following disadvantages arise. Since the overwriting is performed by modulating the magnetic field strength, it is necessary to bring a magnetic field generating mechanism close to a magneto-optical disk to obtain a magnetic field of a sufficient strength. Additionally, with this method, a magnetic field cannot be modulated at high speeds.
In order to overcome such disadvantages, Japanese Publication for Unexamined Patent Application No. 175948/1987 proposes a magneto-optical recording medium having a double-layer structure including a recording layer and auxiliary recording layer, both of which are made by a perpendicularly magnetized film, and a light-intensity modulation overwriting method capable of performing overwriting on the magneto-optical recording medium by modulating only laser power.
With the above-mentioned conventional structures, however, the direction of magnetization of the auxiliary recording layer is changed when performing overwriting. It is thus necessary to execute initialization in order to set the direction of magnetization of the auxiliary recording layer uniform in advance whenever performing the overwriting. An initializing magnetic-field generating mechanism is required in addition to a recording magnetic field generating mechanism, resulting in increases in the size of the magneto-optical recording device and the costs.
Moreover, if the diameter of a recording bit and the interval between recording bits become smaller than the diameter of a light spot formed by focusing laser light on the recording medium, adjacent recording bits enter the light spot. It is therefore impossible to reproduce a single recording bit.